Hydriding/dehydriding properties of nanocrystalline Mg 87Ni 3Al 3M 7 (M = Ti, Mn, Ce, La) alloys prepared by ball milling

Abstract

Nanocrystalline Mg 87Ni 3Al 3M 7 (M = Ti, Mn, Mm; Mm = Ce, La-rich mischmetal) hydrides were synthesized by reactive mechanical milling (RMM) in hydrogen atmosphere. Different degrees of the alloys hydrogenation were achieved varying the time of milling at a hydrogen pressure of 5 atm. The influence of the alloying elements (Ti, Mn, Mm) on the amount of the hydrides formed during RMM as well as on the temperature and enthalpy of hydrogen desorption was determined. The amount of hydride formed during RMM was found to be almost the same in the three alloys (∼35 wt.%); for the Ti containing alloy it was slightly higher. The Ti containing alloy reveals the lowest temperature of hydrogen desorption (∼210 °C) and the hydride in the alloy with Mn decomposes at the highest temperature (∼240 °C) among the three alloys studied. The enthalpy of H-desorption is highest for the sample alloyed with Mm (Ce, La), 70–72 kJ/mol; the Ti and Mn containing magnesium alloys have similar enthalpies of hydride decomposition, 56–60 kJ/mol. The alloys studied reveal higher equilibrium pressures of hydrogen absorption compared to nanocrystalline magnesium, indicating some thermodynamic destabilization of the hydride as a result of the alloying. Hydrogen absorption kinetics were studied at isothermal conditions at different temperatures and an influence of the alloying element was observed. The hydrogen absorption in the Ti containing alloy is substantially faster than in the alloys with Mn and Mm.